Earlier devices exist which fix screws and other threaded members in a substrate such as an agglomerated material including concrete, plaster and epoxy. Two such screw fixing devices are disclosed in U.S. Pat. Nos. 4,085,652 and 5,085,547. The previous screw fixing devices include a sheath embedded into the substrate and into which the screw is threaded.
While the sheaths of the above-referenced devices enable the screw to be easily secured in the substrate, such screw fixing devices have some deficiencies. U.S. Pat. No. 4,085,652 discloses an anchoring device with an overmolded sheath FIG. 1, which has circular ribs and four axial ribs. However, this design of the overmolded sheath makes the screw impossible to remove.
U.S. Pat. No. 5,085,547 improves the screw fixing device by providing a plastic sheath, FIG. 2, into which a screw is threaded and adds a metal sheath surrounding the plastic sheath. The metal sheath is embedded in the substrate and has longitudinal ribs which prevent the sheath from rotating when the screw is threaded into the plastic sheath (FIG. 2).
The design of the sheath in the previous screw fixing devices having two longitudinal ribs which are positioned opposite each other is disadvantageous. The longitudinal ribs can create fragile zones 1, Prior Art FIG. 1, in a substrate such as concrete where moisture and stresses on the screw may cause cracking in the substrate. Because the longitudinal ribs lie in the same plane, the fragile zones in the substrate also lie in the same plane and the forces acting on the screw and sheath will be concentrated in that plane.
Furthermore, the longitudinal ribs of the screw fixing devices extend too close to the top and bottom of the screw fixing sheath. Having the ribs close to the top of the screw fixing sheath causes fragile zones in the weaker top layer of the substrate which is more susceptible to fracturing by forces applied to the screw. Having the ribs extend to the bottom of the sheath below the end of the screw also causes fragile zones at the end area in the sheath.
Another problem with previous screw fixing devices involved the contamination of the interior of the sheath with water and other debris. During manufacturing and shipping of the screw fixing sheath, water and debris could enter the threaded interior of the sheath causing problems when a screw is threaded into the sheath. Further, after the screw has been threaded into the sheath water could seep in causing corrosion and damage from expansion when the water freezes.
Accordingly, what is needed is a screw fixing or anchoring device having a sheath which does not create fragile zones in the substrate where forces may be concentrated in a plane, causing the substrate to fracture. An anchoring sheath must be provided which has at least three ribs spaced around the sheath which do not lie in the same plane, for distributing the forces and preventing the forces from concentrating in a single plane in the substrate and for preventing the sheath from rotating when a screw is threaded into or removed from the sheath. Furthermore, an anchoring sheath is needed which reduces the zones in the substrate which are susceptible to being fractured at the top and bottom of the anchoring sheath. Finally, the screw fixing sheath should have a sealable opening to prevent damage from water and other debris.